拓扑排序

/**
 * Definition for Directed graph.
 * class DirectedGraphNode {
 *     int label;
 *     ArrayList<DirectedGraphNode> neighbors;
 *     DirectedGraphNode(int x) { label = x; neighbors = new ArrayList<DirectedGraphNode>(); }
 * };
 */
public class Solution {
    /**
     * @param graph: A list of Directed graph node
     * @return: Any topological order for the given graph.
     */    
    public ArrayList<DirectedGraphNode> topSort(ArrayList<DirectedGraphNode> graph) {
        // write your code here
        ArrayList<DirectedGraphNode> order = new ArrayList<>();
        if (graph == null) {
            return order;
        }
        //1.count indegree
        Map<DirectedGraphNode, Integer> indegree = getIndegree(graph);
        //2.bfs
        Queue<DirectedGraphNode> queue = new LinkedList<>();
        for (DirectedGraphNode node : graph) {
            if (indegree.get(node) == 0) {
                queue.offer(node);
                order.add(node);
            }
        }
        while (!queue.isEmpty()) {
            DirectedGraphNode node = queue.poll();
            for (DirectedGraphNode neighbor : node.neighbors) {
                //node -> neighbor 
                indegree.put(neighbor, indegree.get(neighbor) - 1);
                if (indegree.get(neighbor) == 0) {
                    queue.offer(neighbor);
                    order.add(neighbor);
                }
            }
        }
        //check 环状依赖
        if (order.size() == graph.size()) {
            return order;
        } else {
            return null;
        }

    }
    private Map<DirectedGraphNode, Integer> getIndegree(ArrayList<DirectedGraphNode> graph) {
        //1.统计indegree,用map装
        Map<DirectedGraphNode, Integer> indegree = new HashMap();
        //初始化
        for (DirectedGraphNode node : graph) {
            indegree.put(node, 0);
        }
        for (DirectedGraphNode node: graph) {
            for (DirectedGraphNode neighbor : node.neighbors) {
                //node -> neighbor
                indegree.put(neighbor, indegree.get(neighbor)+1);
            }
        }
        return indegree;
    }
}